An internal combustion engine comprising fuel injectors positioned so as to directly inject the fuel into the respective combustion chambers is known. In such an engine, it is known that combustion products (i.e. matter derived from the combustion of the fuel) are produced and this combustion products accumulate on a fuel injector wall face (hereinafter, this wall face will be referred to as—injection hole wall face—) of an injection hole area (i.e. an area formed of an interior area of the fuel injection hole of the fuel injector, an exterior area of the fuel injection hole and adjacent to the inlet of the fuel injection hole and an exterior area of the fuel injection hole and adjacent to the outlet of the fuel injection hole). Then, when the combustion products accumulate on the injection hole wall face, even if a command for making the fuel injector inject the fuel having a desired amount is supplied to the fuel injector, the fuel having the desired amount may not be injected from the fuel injector. Then, when the fuel having the desired amount is injected from the fuel injector, the output property and the exhaust gas property of the engine may be decreased. In the fuel injection control device of the engine described in the Patent Document 1, when the amount of the combustion products accumulating on the injection hole wall face (hereinafter, the combustion products accumulating on the injection hole wall face will be referred to as—deposit—and the amount of the deposit will be referred to as—deposit accumulation amount—) is equal to or larger than a base amount, the fuel injection from the fuel injector is controlled so as to separate the deposit from the injection hole wall face.
In the fuel injection device described in the Patent Document 1, the deposit accumulation amount is used for judging if the deposit should be removed from the injection hole wall face. Therefore, in this Patent Document, it is necessary to estimate the deposit accumulation amount. In this regard, when an amount of the fuel actually injected from the fuel injector is referred to as actual fuel injection amount, an amount required as that of the fuel injected from the fuel injector is referred to as required fuel injection amount and a command value supplied to the fuel injector for inject the fuel having the required fuel injection amount from the fuel injector when the deposit accumulation amount is zero is referred to as fuel injection command value, in the Patent Document 1, the deposit accumulation amount is estimated on the basis of the difference between the actual and required fuel injection amounts when the actual fuel injection amount is smaller than the required fuel injection amount from the understanding that when the deposit has accumulated on the injection hole wall face, the actual fuel injection amount becomes smaller than the required fuel injection amount and as the deposit accumulation amount becomes large, the actual fuel injection amount becomes smaller than the required fuel injection amount. It should be noted that in this case, as the difference between the actual and required fuel injection amounts becomes large, it is estimated that the deposit accumulation amount is large.